This invention relates to the processing of cathode ray tubes, and more particularly relates to an improved process in which color tubes for color television are scanned with a weak defocused electron beam.
In the manufacture of color cathode ray tubes for color television and other display applications, various tube processing steps are carried out to insure an acceptable life of reliable operation in the field. This processing begins after assembly of the tube components, and includes: exhausting and baking the tube to evacuate the envelope and outgas the tube and components; flashing a getter onto the internal surfaces of the tube and components to provide continuous gettering of residual contaminants which are outgassed during tube operation; activating the cathodes of the electron gun by heating to promote the formation of low work function species in the emission layer; aging the cathode and lower grid elements of the gun to maintain cathode activation; and high voltage conditioning of the electron gun to remove particles and projections which could lead to interelectrode arcing.
Tube processing is sometimes concluded with a final step of raster scanning the mask and screen of the tube with a focused electron beam, produced using normal gun voltages to simulate operating conditions, for a time sufficient to outgas the scanned surfaces and allow the tube to stabilize prior to shipment to the customer.
It has also been proposed to substitute less precise scanning of a defocused beam for the above raster scanning, in order to avoid the need for sophisticated deflection yokes and associated circuitry. However, raster scanning with a focused beam has the additional advantage of enabling visual inspection for blocked mask apertures.
The rate of outgassing during exhausting and baking is time and temperature dependent, and the throughput demands of the manufacturing process as well as the limited thermal stability of certain tube components make complete outgassing during this stage impractical. Thus, some residual gas and gas-producing contaminants, such as hydrocarbons, remain in the tube after sealing of the exhaust tubulation.
Getter flashing usually introduces additional hydrocarbon contaminants into the tube. These hydrocarbons cannot be effectively adsorbed by the non-bakable barium getters widely used in color television picture tubes. However, during subsequent aging, at least some of these hydrocarbons are dissociated into getterable components, resulting in the reduction of residual gas in the tube to acceptable levels.
It has been found that aging is most effective when the focusing electrode adjacent the lower grid electrodes is included in the aging process, and such aging is referred to herein as "G-3 aging", after the conventional designation of this electrode.
Unfortunately, when the focusing electrode is included in aging, a condition known as "dark center cathode" can result, which by analysis has been found to be due to a carbon deposit in the center of the emissive layer of the cathode. Surprisingly, this deposit does not materially reduce cathode emission. However, it does restrict emission to the area of the perimeter of the emissive layer, resulting in a hollow beam which interferes with proper focusing and image resolution at the screen.
In allowed U.S. patent application Ser. No. 145,637, filed Jan. 11, 1988, and assigned to the present assignee, the cause of dark center cathodes was postulated to be due to the successively higher voltages which are impressed on the G1, G2 and G3 electrodes during aging, causing the electrons emitted from the cathodes to be focused into an electron beam which dissociates residual hydrocarbons present in the tube after exhausting, baking and getter flashing, and results in the formation of a beam of positive carbon ions which travel in the reverse direction from the electron beam and are deposited on the cathode.
It was shown that reducing the potential of the G3 electrode during aging to a critical level above a threshold needed for effective aging, but sufficiently below the potential of the G2 grid to create a potential barrier and prevent the positive beam from reaching the cathode, significantly reduces the incidence of dark center cathodes while substantially retaining the benefits of G3 aging.
However, even with the above G3 aging, some incidence of dark center cathode can still occur, and residual gas levels can still be higher than desired.
Accordingly, it is a principal object of the present invention to reduce hydrocarbons present in a color cathode ray tube after getter flashing, without depositing carbon on the cathodes.
It is another object of the invention to reduce hydrocarbons below the level obtained by the improved G-3 aging process described above, without depositing carbon on the cathodes.
Still other objects of the invention are to reduce residual gases present after getter flashing, and to reduce these residual gases below the level obtained by the improved G-3 aging process.